As far as I have understood this user has multiple calendars.. These should appear underneath the "Calendar" in Outlook....
What I think is happening is that your users when on the iPhone is viewing one of these calendars or is on the "All" setting and the default calendar is set to a different calendar, and adding an item.. It will then add the item to that one calendar, therefore if you look in Outlook on one of ht eother calendars then you wont see the item.
Confusing the matter is that Outlook has a "All" view that shows all of the items from all of the Calendars in outlook, in one view, Outlook doesnt do this.
Solutions..
- Copy all events into the root calendar (might be a bit drastic! and as long as the user understands multiple calendars it will work) and delete all the other cals
- Change the default calendar to the one that he most regulary adds items:
- Settings
- Mail,Contacts and Calendars
- Scroll to the bottom
Hopefully it is that..>!